The purpose of this project is to determine the mechanism by which macrophages distinguish between adult and senescent self. Experiments indicate that human macrophages make this distinction on the basis of selective IgG attachment to the surface of senescent cells. The objective of this project is to isolate and characterize the regulatory IgG. The antibody eluted from senescent cells was shown to be (1) an IgG without other immunoglobulins by immunodiffusion, immunoelectrophoresis, and acrylamide gel electrophoresis, (2) polyclonal, and (3) a regulatory autoantibody.